1. Field of the Invention
The present invention relates to a switching power supply device capable of actuating an overload protection after a delay is developed for a predetermined time since an overload is detected.
2. Background Art
Conventionally, in a switching power supply, an overload protection or an overcurrent protection function has been proposed in order to prevent an excessive current from passing through an output portion and a device connected to the output portion in an abnormal state of the device.
In the protection, a method for detecting an overload and an overcurrent state becomes important from the technical viewpoint. Examples of the method for detecting an overload and an overcurrent state include a method for detecting an overload by a drop in voltage at an output portion of a switching power supply during the overload and a method in which a current passed through a switching element becomes a predetermined value or more, thereby detecting the overload.
For example, Japanese Patent Laid-open Publication No. 06-197530 discloses a flyback type switching power supply device in which a third winding of a transformer is utilized. The method will briefly be described with reference to a power supply circuit shown in FIG. 21. A primary winding 31a is connected in series to a switching element 1, a secondary winding 31b supplies power to an output portion, and an auxiliary winding 31c which is a third winding is provided independently of the primary winding 31a and the secondary winding 31b. The auxiliary winding 31c is equal in polarity to the secondary winding 31b. A feature of an auxiliary winding smoothing voltage (a VCC terminal voltage at a control circuit 2) which is a smoothed voltage at the auxiliary winding 31c is that the auxiliary winding smoothing voltage rises with the increase of output power. Utilizing the feature, the auxiliary winding smoothing voltage becomes a predetermined value or more, thereby allowing detection that the output power becomes a predetermined value or more. Therefore, the flyback type switching power supply device detects the overload state.
The flyback type switching power supply device disclosed in Japanese Patent Laid-open Publication No. 06-197530 has an advantage that the output power actuating the protection has weak dependence on an input voltage. The advantage is not lost in not only PWM control which is operated at a constant oscillation frequency, which will be described in the present invention, but also an RCC type power supply in which the oscillation frequency changes depending on the input voltage.
On the other hand, for a power supply in which large power needs to be instantly supplied to the output portion while the output power is maintained for a predetermined time or more, there is proposed a power supply in which the protection is actuated after a predetermined delay time is provided since an overload is detected, in order to deal with the case in which the overload protection needs to be actuated, or in order to prevent a malfunction of the overload protection.
Japanese Patent Laid-open Publication No. 2004-48884 discloses a circuit in which a hold circuit holding an overcurrent detection signal for a predetermined time is provided, whereby the protection is actuated after a predetermined delay time elapses since the overload is detected.
However, in the switching power supply device disclosed in Japanese Unexamined Patent Publication No. 06-197530 in which an overload is detected by the rise of the auxiliary winding smoothing voltage, it is necessary to take the following two methods in order to provide the delay time after the overload is detected as described above:
1) A smoothing capacitor 4 of the auxiliary winding is increased, thereby delaying the rise of the auxiliary winding smoothing voltage after the output power is increased; and
2) A circuit generating the delay time is added separately.
In the method 1), a large smoothing capacitor is required to sufficiently delay the rise of the auxiliary winding smoothing voltage, and actually a large aluminum electrolytic capacitor is required, which results in increased occupying space and cost.
In the method 2), similarly to the switching power supply device disclosed in Japanese Unexamined Patent Publication No. 06-197530, the addition of a complicated circuit is required. Therefore, the increased occupying space and cost are unavoidable in the case where the circuit is configured by discrete components. Even if a commercially available control IC is used as the control circuit 2 of the switching element 1, it is necessary to use a semiconductor IC having the function, which results in a decreased degree of freedom in design.